1. Technical Field
The disclosed embodiments relate to driving a mixer, and more particularly to diving a mixer in the transmit chain of a wireless transmitter.
2. Background Information
In many radio transmitters, such as radio transmitters of cellular telephone handsets, information to be communicated is modulated onto a carrier for transmission. There are many complex modulation schemes that can be employed, but most of these schemes as currently practiced in cellular telephones can be categorized as involving one of two general approaches. In a first approach, a Voltage Controlled Oscillator (VCO) outputs a high frequency signal. The high frequency signal is then amplified and transmitted from an antenna. The VCO is directly modulated with intelligence information. A Digital-to-Analog Converter (DAC) may be used to supply a control signal to the VCO such that the VCO output signal is modulated to include the intelligence information. This first approach has certain advantages and disadvantages. In a second approach, a VCO is used but this VCO is not directly modulated with intelligence information. Rather, a relatively stable and fixed-frequency VCO output signal is supplied to a mixer. In addition, a lower frequency signal that includes the modulation intelligence information is supplied to the mixer. The lower frequency signal (also referred to as a baseband signal) is typically generated using a DAC. The mixer multiplies the VCO output signal by the baseband modulation intelligence information signal, thereby generating a higher frequency signal at about the frequency of the LO signal that includes the intelligence information. This higher frequency signal is then amplified and is transmitted from an antenna. This second approach also has certain advantages and disadvantages.
FIG. 1 (Prior Art) is a simplified diagram of a circuit that employs the second approach. Local oscillator 1 includes a Phase-Locked Loop (PLL) (not shown) which in turn includes a VCO (not shown). Local oscillator 1 generates a signal referred to here as a Local Oscillator (LO) signal. This LO signal is essentially an output of the VCO. The LO signal is supplied to one input of a mixer 2. A digital intelligence signal 3 includes intelligence information to be communicated. Digital signal 3 is converted into analog form by a DAC 4 such that an analog intelligence baseband signal is generated. This analog signal is filtered by filter 5 and is supplied to a second input of mixer 2 as an intelligence baseband signal BB. Mixer 2 multiplies the intelligence baseband signal BB with the LO signal to upconvert the intelligence signal in frequency. The upconverted signal 6, that includes the intelligence information, is then amplified by a driver amplifier 7 and a power amplifier 8 and is transmitted from an antenna 9.
FIG. 2 (Prior Art) is a diagram that illustrates a problem associated with the circuit of FIG. 1. In the illustrated example, the LO signal is of frequency 1 GHz and the baseband intelligence signal BB is of frequency 100 KHz. Mixer 2 is not an ideal circuit component, but rather it exhibits non-ideal characteristics. The signal 6 output by mixer 2 actually includes a signal 10 at the frequency of the fundamental of LO signal (1 GHz), as well as signals 11 and 12. The signal 11 has a frequency of three times the fundamental frequency. The signal 12 has a frequency of five times the fundamental frequency. Signals 11 and 12 are two of the odd harmonics of the fundamental signal. Although only two of these harmonics are illustrated, in actuality there are additional higher order odd harmonics that are also generated. In addition to generating the signals 10-12 at the fundamental frequency and at the odd harmonic frequencies, mixer 2 also outputs an upconverted version 13 of the intelligence signal. In addition, if the frequency of this signal 13 is the fundamental frequency plus the frequency of the baseband signal (1 GHz plus 100 KHz), then mixer 2 will also output versions 14 and 15 of the intelligence signal. Version 14 is located at a frequency of the third harmonic minus the frequency of the intelligence signal. In the example of FIG. 2, this frequency is 3 GHz minus 100 KHz. The mixer 2 also outputs version 15 of the intelligence signal at a frequency of the fifth harmonic plus the frequency of the intelligence signal. In the example of FIG. 2, this frequency is 5 GHz plus 100 KHz. In this pattern, the mixer outputs multiple versions of the intelligence signal, where the versions alternate in frequency positions above and below the odd harmonics of the fundamental, as the spectral components of the mixer are considered going up in frequency. The frequency components of signal 6 are illustrated in the left portion of FIG. 2.
Then, in addition to mixing, the practical circuit of FIG. 1 involves amplification of the mixer output signal 6. Practical amplifiers are non-linear to some extent. Non-linearity in the amplification stages 7 and 8 gives rise to intermixing of the various frequency components of signal 6. As a result of this intermixing, a version of the signal 14 will be folded down in frequency and will appear in the amplifier output as signal 16. The right portion of FIG. 2 illustrates the result of intermixing and the generation of signal 16. As illustrated, the frequency of signal 16 is close to the fundamental frequency of the LO.
FIG. 3 is a diagram that illustrates the right portion of FIG. 2 in further detail. In order to maximize the network capacity for cellular telephone protocols, there are often stringent requirements on how much energy a transmitter can transmit in and around an allotted band. In the example set forth here, the allotted band 17 extends from 1 GHz minus 100 KHz to 1 GHz plus 100 KHz. The folded down signal 16 appears slightly outside this band at a frequency of 1 GHz plus three times 100 KHz. In addition, there are requirements that define the maximum amount of power that the transmitter can transmit at each frequency extending away from allotted band 17 with increasing frequency, and extending away from allotted band 17 with decreasing frequency. Lines 18 and 19 identify these limits on transmit power and are referred to as a transmit mask. Care should be taken to ensure that folded down signal 16 is not of such a large magnitude that it violates the transmit mask.
Several techniques can be employed to ensure that signal 16 does not violate transmit mask requirements. For example, very large amplifiers can be used to realize the driver amplifier 7 and power amplifier 8 stages. Generally, the non-linearity of an amplifier increases as the amplifier is driven harder. If a small amplifier is driven harder to generate more gain such that an output signal of a desired power is generated, then the smaller amplifier will typically exhibit more non-linearity. If, however, a relatively large amplifier is provided to generate the output signal of the desired power, then the amplifier can generally be made to exhibit less non-linearity. Providing such a large amplifier is, however, expensive and/or consumes an undesirably large amount of power.
Rather than oversizing the amplifier in this way, another technique involves making an amplifier that does not output third harmonic components. Such an amplifier can be made using multiple stages, where each stage includes an amplifier that is not overdriven. Each stage therefore can be made to exhibit minimal non-linearity. The signal output from a stage is filtered to eliminate third harmonic components before the filtered signal is supplied to the input of the next amplification stage. Unfortunately this multi-stage technique can introduce an undesirable amount of noise into the amplified signal. In some cellular telephone standards, not only is it prohibited for the transmitter to inject too much power into the region of the allotted band of an adjacent device, but also the transmitter is prohibited from introducing too much noise into a receive band. This receive band is identified by “RX” in FIG. 3. Generally each amplifier stage adds an amount of noise. The accumulation of noise from the many amplifier stages may be so great that receive band noise requirements are violated.
Solutions to these problems are sought.